DESCRIPTION: (applicant's abstract) Biochemistry revolves around the chemistry of carbon, but many other elements play vital roles. Sulfur is such an element, with roles in structure, catalysis and metabolism in all organisms. Specifically in humans, homocysteinemia, together with low S-adenosyl methionine levels, has been implicated in coronary artery disease and in atherosclerotic disease. Our goal is to develop sulfur K-edge X-ray absorption near-edge spectroscopy so that it can be used to monitor and to improve our understanding of sulfur metabolism in living systems. There are currently no effective spectroscopic probes of sulfur in biological systems. X-ray absorption spectroscopy will detect all chemical forms of sulfur in a sample, in solid, in aqueous solution or in any other form, and different forms of sulfur display strikingly different spectra. No pre-treatment is needed and the technique is at least potentially non-destructive. The ultimate potential of the work we propose herein is to provide an in vivo probe of metabolic status. To this end, we propose the following specific aims for this initial study: 1. Generate a library of sulfur spectra of a range of biologically significant compounds under a range of physiologically relevant conditions. 2. Develop statistically rigorous curve-fitting routines for evaluation of complex mixtures, and quantitatively evaluate particle size and concentration effects. 3. Evaluate the potential problems from radiation damage, including the requirement for cryo-protection of the sample. 4. Validate our approach by initial studies of simple biological systems where sulfur plays very major roles in metabolism. 5. Apply the developed techniques to more challenging eukaryotic systems. This latter aim may not be achieved in the initial period of the grant.